Published on Web 11/22/2002
Hydrophobic Interaction and Hydrogen Bonding Cooperatively Confer a
Vancomycin Hydrogel: A Potential Candidate for Biomaterials
Bengang Xing,† Chun-Wing Yu,† Kin-Hung Chow,‡ Pak-Leung Ho,‡ Degang Fu,† and Bing Xu*,†
Department of Chemistry, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Hong Kong,
and Center of Infection and Department of Microbiology, Faculty of Medicine, The UniVersity of Hong Kong,
Pokfulam Road, Hong Kong
Received September 13, 2002
This paper reports the first antibiotic gelatorsvancomycin-pyrene
(1)sthat forms hydrogels via hydrophobic interaction and hydrogenbonding-promoted self-assembly in water. Hydrogels, formed by
three-dimensional, elastic networks whose interstitial spaces are
filled with water, present many useful properties (e.g., response to
the external stimuli) and applications (e.g., gel electrophoresis,
chemical sensing, drug delivery, as a biointerface, and as actuators).1,2 Biopolymers (e.g., collagens,3 polysaccharide,4 etc.) and
hydrophilic synthetic polymers (e.g., polyacrylamide2,5 and polypeptides6) have been successfully employed to form hydrogels.
Nonpolymeric hydrogelators, however, are rare despite that their
counterpartsssmall molecular organogelators7,8shave expanded
rapidly and received intensive studies in the past decade. Recently,
Hamilton,9 Shinkai,10 Zhang,11 and others12 have reported low
molecular weight hydrogelators that form hydrogels via carefully
balancing the hydrophobic interactions and hydrogen bonds in water
to induce aggregations of those small molecules. Their results
inspired us to design and synthesize hydrogelators based on
antibioticssan important class of biomoleculessin the hope of
developing biomaterials that can treat infectious wounds, serve as
an antiseptic matrix, and provide a new way of drug delivery.13
We chose vancomycin (Van), one of the most important
antibiotics, as the platform to make the hydrogelators because of
(1) its clinical significance in treating Gram-positive bacterial
infections;14 (2) its relatively easy synthetic modifications;15 and
(3) its strong tendency to form multiple hydrogen bonds with
suitable substrates or itself in aqueous solution, as revealed by Wash
et al. in the decipherment of the molecular logic of vancomycin
resistance enterococci (VRE),16 by Williams et al. in the elucidation
of binding mode of Van,17,18 and by Whitesides et al. in the studies
of multivalency of Van.19 Learning from the principles developed
in the study of low molecular weight organogelators,7,20 we
successfully generated a hydrogelator based on Van by introducing
a pyrene group to the C-terminal of the backbone of Van. Our
results indicate that the π-π stacking and intermolecular hydrogen
bonding in water provide driving forces to form a noncovalent
polymer of 1, which is primarily responsible for the gelation. We
believe that such an approach, which eliminates the biologically
inactive molecules (e.g., cross linked polyacrylamide, etc.) in
conventional hydrogels, could lead to a new kind of biomaterial
for useful applicationssfor example, controlled releases of therapeutics or surface coatings of medical devices.
Figure 1 shows the chemical structure of 1 and the picture of
the hydrogel (formed by adding 6.5 mg of 1 into 1.8 mL of water,
corresponding to ∼0.36 wt % (2.2 mM) of the gelator and ∼23 000
* To whom correspondence should be addressed. E-mail: chbingxu@ust.hk.
† The Honk Kong University of Science and Technology.
‡ The University of Hong Kong.
14846
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J. AM. CHEM. SOC. 2002, 124, 14846-14847
Figure 1. The structure of 1 and the optical image of the hydrogel of 1
(0.36 wt %) (taken by a flatbed scanner when the vial lay horizontally).
Figure 2. (a) Emission spectra of Van-pyrene (1) hydrogels at two different
concentrations (λexcitation ) 330 nm). (b) Circular dichroism spectra of an
aqueous solution of Van (2.2 mM) and Van-pyrene (1) hydrogel (2.2 mM).
water molecules/gelator molecule). Figure 2a shows the emission
spectra of the hydrogels of 1 at two different concentrations. The
broad band of the emission (λmax ) 460 nm), resembling the
emission of pyrene excimer (λmax ) 480 nm),21 indicates that the
pyrenes of 1 dimerize exclusively via π-π stacking in the gel.
Figure 2b shows the circular dichroism22 (CD) spectra of Van (2.2
mM) and the hydrogel (2.2 mM). The large Cotton effect at 220
nm in the spectrum of Van originates from the peptidal backbone
of Van. The subsequent decrease of its intensity in the gel phase
indicates that the peptidal backbones associate in head-to-tail fashion
by (possibly) four hydrogen bonds, as indicated in the crystal
structure23 and solution structure17 of Van. The head-to-tail arrangement of the peptidal leads to meso orientation of the backbones
and results in smaller CD signals at 220 nm in the gel. The intensity
increases at 285 and 340 nm (the induced circular dichroism)
suggest that the biphenyl and pyrene moieties adopt helical
arrangements in the polymer of 1. The helical arrangement was
later confirmed by transmission electron micrography (Figure 3c),
which reveals that the polymers aggregate into superhelices with
the diameter of ∼25 nm, similar to the observations in many other
systems.24
10.1021/ja028539f CCC: $22.00 © 2002 American Chemical Society
COMMUNICATIONS
L.; Mortensen, P. B.; Brondsted, H. Eur. J. Pharm. Sci. 1995, 3, 329337. Osada, Y.; Gong, J. P. AdV. Mater. 1998, 10, 827-837. Weissman,
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Figure 3. (a) Illustration of π-π stacking and multiple hydrogen bonding
(the dotted lines) in the polymer of 1, (b) one of the possible conformations
of the helix of 1, and (c) the transmission electron micrograph (TEM) of
the helical fibers in the hydrogels.
The differential scanning calorimetry (DSC) measurement indicates that the formation of the gel is thermally reversible with the
gel-to-sol transition temperature (TGS) at ∼73 °C and ∆HGS of
∼7.05 kcal/mol, which corresponds to the entropy of 20.4 cal/mol.
The sum of ∆G’s, being considered separately, of dimerization of
Van (∼ -3.54 kcal/mol)17 and π-π stacking of pyrenes (∼ -2.9
kcal/mol),25 however, is not enough to compensate for the entropy
loss due to immobilization of water. Therefore, cooperative
interactions (e.g., ∼0.99 kcal/mol in the case of pyrene-peptide
conjugates25) should exist to provide additional driving force for
the gelation. Such an interaction may originate from the conformational change of Van-pyrene upon π-π stacking of the pyrenes.
On the basis of the CD, fluorescent spectra, and electron
microscopy of the gel, we suggest that the Van-pyrene self-assemble
to form a helical polymer, whose molecular superstructure is
proposed in Figure 3a and b. The small-angle X-ray diffraction of
the hydrogel exhibits a broad peak at ∼25°, suggesting that the
fibers may form random networks rather than an ordered phase.
The absence of higher order peaks also suggests that a columnar
phase due to π-π stacking of pyrene is a unlikely scenario,
prevented by the steric congestion imposed by Van.
We found, in a separate research, that 1 is unexpectedly potent
(0.125-2 µg/mL, being 8- to 11-fold dilutions lower than the
corresponding vancomycin) against VRE (2 VanA-positive Enterococcus faecalis, 4 VanA-positive E. faecium, 4 VanB-positive E.
faecium).21 The strong tendency to polymerize and the unexpected
potency of 1 also lead us to speculate that 1 might aggregate into
polymer-like structures at the cell surface when its local concentration is high, and we are working on confirming this hypothesis.
In summary, we have demonstrated a new kind of small
molecular hydrogelators based on an important member of antibiotics. Such antibiotic hydrogels will provide a new kind of biomaterial.
Acknowledgment. This work was partially supported by the
Research Grant Council of Hong Kong, a Direct Allocation Grant
(HKUST), a DuPont Young Faculty Grant (for B.X.), and a Grant
from the University Development Fund (HKU). We thank Dr.
Yesha Zheng for the TEM measurement.
Supporting Information Available: Details of the synthesis and
the in vitro test of 1 (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
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